1. Field of the Invention
The present invention relates to an improvement in an image capturing apparatus having an image blurring correction function and zoom position change function.
2. Description of the Related Art
Generally, an image capturing apparatus obtains image data with a periphery including the four corners which is darker than the center of an image. Especially, a phenomenon in which the light amount at the periphery becomes lower than that of the center is called shading. As the rate of decrease in light amount from the center to the periphery increases, or as the four corners form more asymmetric shading, i.e., a variation in their luminances increases, the quality of image data degrades. A lens exhibits a unique degree of decrease in light amount. This property causes a variation in luminance. The degree of decrease in light amount changes also depending on the focal length, i.e., zoom position if the lens is a zoom lens.
For example, the wide-angled side may exhibit a highest rate of decrease in the light amount at the lens periphery, and the telephoto side may exhibit a very low rate of decrease (see FIG. 3A). A noticeably high rate of decrease in the light amount at the periphery can be caused by a mechanical error or play of a lens barrel which houses lenses, resulting in an increase in variation in the luminances at the four corners (see FIG. 3B).
An apparatus using the arrangement of an image blurring correction function is available as a technique for reducing a variation in the luminances at the four corners of an image (see Japanese Patent Laid-Open No. 2004-40298).
Along with the recent increase in magnification and miniaturization of image capturing apparatuses, a demand has arisen for installing a function of correcting image blurring due to a camera shake of an image capturing apparatus. In fact, the number of image capturing apparatuses with this function is increasing. The image blurring correction mechanism includes a sensor means for mainly detecting shaking such as a camera shake of an image capturing apparatus, a calculation means for converting the resultant shaking amount into a shaking cancel amount, and a correction means such as a correction lens (to be referred to as a shift lens hereinafter) for eliminating image blurring. A shift lens operates in a plane perpendicular to the image sensing optical axis to eliminate blurring of an image formed on an image sensor.
By adjusting the center point of a shift lens at a zoom position having a highest rate of decrease in the light amount at the periphery, a variation in the luminances at the four corners can fall within a predetermined level in the entire zoom area. The adjusted center point may be different from the center of the mechanical driving range of the shift lens.
Assume that a lens to be used has a wide-angled side with a highest rate of decrease in the light amount at the lens periphery and has a telephoto side with a very low rate of decrease, and a position at which a variation in the luminances at the four corners on the wide-angled side is adjusted serves as a center point fixed in the entire zoom area. As the shift between a center point at which a variation in the luminances at the four corners is adjusted and the center point of the mechanical driving range increases, a change in angle of view increases at the time of zoom driving to the telephoto side because the center of the angle of view must originally be the center point of the mechanical driving range. Especially, the zoom position on the telephoto side suffers a noticeably large change in angle of view. This results in image data with an angle of view which is not intended by a photographer (with an error of the angle of view at the time of zoom driving), depending on the zoom position. Furthermore, the visibility of a live image at the time of zoom driving suffers.
A wider dynamic range of the driving of a shift lens is advantageous to image blurring correction. This is because a wider driving range of a shift lens decreases the property change amount in control when a camera shake and abrupt large shaking are detected simultaneously. This allows high-speed, precise image blurring correction. A wider original driving dynamic range is also advantageous to assure a control range sufficient to decrease a variation in center point of a shift lens due to temperature change. For this reason, image blurring correction becomes optimal when the center point of a shift lens serves as the center of the mechanical driving range. However, optimal image blurring correction control sometimes fails when the center point of a shift lens serves as the luminance center in the entire zoom area.